Research brief: What caused the rapid decline in Antarctic sea ice in 2016?

IAntarctic sea ice extent underwent a rapid decline in the spring of 2016 and is still well below average now. CLEX researchers have tied the decline to natural variability of both the atmosphere and ocean in two articles published in Nature Communications this month.

Antarctic
sea ice extent (the area of the Southern Ocean covered by sea ice) had been
slowly expanding since satellite monitoring began in the late 1970s. So when it
dramatically declined in late 2016 it prompted many questions. What caused the
decline and what has sustained it since? Was it the beginning of a longer-term
decline or a temporary shift? And what role might human-induced climate change
have played?

Using
statistical and climate model analysis, we identified the dominant role of the
very warm sea surface temperatures in the tropical eastern Indian Ocean in the
spring of 2016 in driving wind patterns over the Southern Ocean. These
northerly winds acted to push the sea ice back towards Antarctica as well as
melt it via warm and moist airflow. Later in 2016 the normal westerly winds
that encircle Antarctica also weakened, leading to further sea ice declines. We
found that the weakening of these winds began in the stratosphere.

But
what has sustained the below average sea ice cover since? Again, our research
points to the role of the tropics. We argue that tropical Pacific sea surface
temperature variability drove stronger
than usual westerly winds over the previous 15 years that acted to bring warm
subsurface ocean water slowly towards the surface in the sea ice region. The
warmer ocean has maintained the reduced extent of Antarctic sea ice since.

Our results suggest that the rapid sea ice declines in 2016 were largely due to natural variability of the climate system, but a role for climate change cannot be ruled out.